Iraq's president named a new prime minister to replace Nuri al-Maliki on Monday, urging him to form a broad government that can stem communal bloodshed, but it was unclear whether Maliki would bow to U.S. and Iranian pressure to step aside.

A Shi'ite Muslim blamed by erstwhile allies in Washington and Tehran as well as Baghdad for driving the alienated Sunni minority into revolt, Maliki deployed loyal militias and special forces in the capital on Monday after making a defiant speech accusing the head of state of abusing the constitution.

Militants from the Islamic State, who routed Maliki's army in the north in June, made new gains over Kurdish forces despite three days of U.S. air strikes and Baghdad, long braced for the Sunni fighters to attack the city, was now tensing for possible clashes between Maliki and rivals within the Shi'ite majority.

There was no immediate reaction from Maliki to the naming of Haider al-Abadi as prime minister. However, Maliki's son-in-law, a close political ally, told Reuters that he would seek to overturn the nomination in the courts.

President Fouad Masoum asked Abadi, a leader of Maliki's Islamic Dawa Party, to lead an administration that can win the support of a parliament elected in April. In remarks broadcast on television, Masoum, an ethnic Kurd, urged him to "form a broader-based government" over the next month.

Abadi himself, who spent decades in exile in Britain during the rule of Sunni dictator Saddam Hussein, urged national unity against the "barbaric" Islamic State, which has driven tens of thousands from their homes as it swept Baghdad's troops from the north and west to consolidate a "caliphate" in Iraq and Syria.

"We all have to cooperate to stand against this terrorist campaign launched on Iraq and to stop all terrorist groups," he said in broadcast remarks after meeting Masoum.

Maliki, 60, who emerged from obscurity to become prime minister in 2006 under U.S. occupation, may not go quietly.

"We will not stay silent," his son-in-law Hussein al-Maliki said. "The nomination is illegal and a breach of the constitution. We will go to the federal court to object."

U.S. WARNING

After Washington endorsed Masoum's attempts to break three months of post-election political deadlock that have hamstrung Baghdad's response to the Islamic State, Secretary of State John Kerry called on Maliki not to resort to force or "stir the waters" when Iraqis were seeking a change of leader.

In pointed remarks, he said: "The government formation process is critical in terms of sustaining stability and calm in Iraq and our hope is that Mr. Maliki will not stir those waters.

"There will be little international support of any kind whatsoever for anything that deviates from the legitimate constitution process that is in place and being worked on now."

As police and elite armed units, many equipped and trained by the United States, locked down the capital's streets, Kerry added: "There should be no use of force, no introduction of troops or militias in this moment of democracy for Iraq."

Serving in a caretaker capacity since the inconclusive election on April 30, Maliki has defied calls by Sunnis, Kurds, fellow Shi'ites, regional power broker Iran and Iraq's top Shi'ite cleric to step aside for a less polarising figure.

"Maliki knows it is very difficult to gain a third term and is playing a high-stakes game to try and ensure his authority and influence continue into the new government, despite who may officially become prime minister," said Kamran Bokhari, a Middle East specialist at analysis firm Stratfor.

Washington is losing patience with Maliki, who has placed Shi'ite political loyalists in key positions in the army and military and drawn comparisons with Saddam, the man he plotted against from Iranian exile for decades.

Before Abadi's nomination, a U.S. State Department spokeswoman reaffirmed support for a "a prime minister who can represent the aspirations of the Iraqi people by building a national consensus and governing in an inclusive manner".

"We reject any effort to achieve outcomes through coercion or manipulation of the constitutional or judicial process," she said in a statement, adding that the United States "fully supports" Masoum as guarantor of Iraq's constitution.

MILITARY AID

U.S. President Barack Obama has urged Iraqi politicians to form a more inclusive government that can counter the growing threat from the Islamic State, though he has rejected calls in some quarters for a return of U.S. troops other than in the form of several hundred military advisers sent in June.

The group, which sees Iraq's majority Shi'ites as infidels who deserve to be killed, has ruthlessly moved through one town after another, using tanks and heavy weapons it seized from soldiers who have fled in their thousands.

On Monday, police said the fighters had seized the town of Jalawla, 115 km (70 miles) northeast of Baghdad, after driving out the forces of the autonomous Kurdish regional government.

Washington and its European allies are considering requests for more direct military aid from the Kurds, who have themselves differed with Maliki over the division of oil resources and took advantage of the Islamists' advance to expand their territory.

On Sunday, a government minister said Islamic State militants had killed hundreds of people from the small, Kurdish-speaking Yazidi religious sect, burying some alive and taking women as slaves. No confirmation was available of the killings.

Thousands of Yazidis have taken refuge in the past week on the arid heights of Mount Sinjar, close to the Syrian border.

The bloodshed could increase pressure on Western powers to do more to help those who have fled the Islamic State's offensive. They have already dropped supplies and U.S. aircraft have been bombing the militants since Friday.